Functional nanoscale metal oxides for stable metal single atom and cluster catalysts

ABSTRACT

A nanocomposite catalyst includes a support, a multiplicity of nanoscale metal oxide clusters coupled to the support, and one or more metal atoms coupled to each of the nanoscale metal oxide clusters. Fabricating a nanocomposite catalyst includes forming nanoscale metal oxide clusters including a first metal on a support, and depositing one or more metal atoms including a second metal on the nanoscale metal oxide clusters. The nanocomposite catalyst is suitable for catalyzing reactions such as CO oxidation, water-gas-shift, reforming of CO2 and methanol, and oxidation of natural gas.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No.62/876,437 entitled “FUNCTIONAL NANOGULES FOR STABLE METAL SINGLE ATOMAND CLUSTER CATALYSTS” and filed on Jul. 19, 2019.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under 1465057 awarded bythe National Science Foundation. The government has certain rights inthe invention.

TECHNICAL FIELD

This invention relates to functional nanoscale metal oxide for stablemetal single atom and cluster catalysts.

BACKGROUND

Supported metal catalysts are used in many important catalytic reactionsfor producing chemicals and energy or for environmental remediation.Since catalysis is a surface reaction process, the use of smaller metalparticles can save cost and/or yield better catalyticselectivity/activity. However, smaller metal particles, clusters orsingle atoms are not thermodynamically stable and usually sinter to formlarger particles during a catalytic reaction, especially at elevatedtemperatures and under a reducing environment. For relatively hightemperature catalytic reactions (e.g., control of emissions fromautomobiles and stationary sources), the high-surface-area supportsgenerally need to be able to resist sintering at high temperatures. Suchinert refractory support materials (e.g., SiO₂, Al₂O₃, etc.) have beenused as sintering resistant high-surface-area supports, but typically donot strongly anchor metal clusters or single metal atoms.

SUMMARY

As described herein, nanoscale metal oxides are used to strongly bindthe metal atoms or clusters and high-surface-area refractory supports. Afacile and scalable wet chemistry synthesis approach is developed todeposit reducible nanoscale metal oxides, or “nanoislands,” ontohigh-surface-area refractory oxide supports, and preferentially depositmetal atoms or clusters onto only the reducible metal oxide nanoislandsbut not onto the high-surface-area refractory oxide support surfaces.Such supported metal atom or cluster catalysts proved extremely stableand active for a variety of catalytic reactions. The reducible metaloxide nanoislands localize metal atoms or clusters to prevent sintering,and provide desirable catalytic function(s) during a targeted catalyticreaction.

This disclosure relates to the use of nanoscale metal oxides as“nanoislands” that can bind strongly to metal atoms or clusters as wellas high-surface-area refractory supports. The metal oxide nanoislandstypically have a dimension (e.g., diameter or height) of 0.5 nm to 10nm. In some cases, the metal oxide nanoislands having a dimension of 0.5nm up to 3 nm are referred to as “nanoglues,” while the metal oxidenanoislands having a dimension of 3 nm to 10 nm are referred to as“nanoparticles” or “nanocrystals.” In some embodiments, metal atoms,clusters, or particles are deposited on refractory support surfaces onwhich a nanoscale metal oxide has been dispersed, thereby stronglybinding the metal species. The individual metal oxide nanoislands areisolated from each other. This approach allows scalable manufacturing ofsintering resistant atomically dispersed metal catalysts. Many types ofreducible metal oxides may be utilized to produce metal oxidenanoislands. The choice of suitable metal oxides depends on the specificcatalytic reaction of interest. In this case, the nanoislands possesstheir own function (e.g., providing readily available active surfaceand/or lattice oxygen species) during a catalytic reaction. To constructa stable atomically dispersed or cluster catalyst, any metal species,including noble metal species, can be used. The nanoislands can includeone or more reducible metal oxides. Suitable high-surface-arearefractory supports include silica, alumina, magnesia, zirconia,combinations of these, and other appropriate materials such ascordierite and perovskite-type oxides. In one example, CeO_(x), a highlyreducible metal oxide, is used as a nanoisland, and high-surface-areaSiO₂ is used as the support material. Through a facile and scalable wetchemistry synthesis method, Pt single atoms, clusters, or nanoparticlesare preferentially deposited onto the CeO_(x) nanoisland to produce aPt₁/CeO_(x)—SiO₂ single-atom catalyst or Pt_(n)/CeO_(x)—SiO₂ clustercatalyst. Such catalysts have proven to be extremely active and stablefor CO oxidation reaction, even at temperatures below 150° C.

In a first general aspect, a nanocomposite catalyst includes a support,a multiplicity of nanoscale metal oxide clusters coupled to the support,and one or more metal atoms coupled to each of the nanoscale metal oxideclusters.

Implementations of the first general aspect may include one or more ofthe following features.

The support may include a refractory material having a surface area ofat least 50 m²/g or at least 100 m²/g. Suitable examples of supportmaterials include silica, alumina, magnesia, zirconia or any combinationthereof. The support can be powdered.

The nanoscale metal oxide clusters typically have a dimension in a rangeof 0.5 nm to 10 nm. The nanoscale metal oxide clusters may includeCeO_(x), COO_(x), FeO_(x) TiO_(x), CuO_(x), NiO_(x), MnO_(x), NbO_(x),VO_(x), ZrO_(x), or any combination thereof. In some cases, thenanoscale metal oxide clusters include CeO₂, Co₃O₄, Fe₂O₃, TiO₂, CuO,NiO, MnO₂, Nb₂O₅, V₂O₅, ZrO₂, or any combination thereof.

The one or more metal atoms can include metal clusters (e.g., metalclusters having 2 to 100 metal atoms). In some cases, the one or moremetal atoms independently include one or more transition metal atoms,one or more precious metal atoms, or both. Examples of suitable metalatoms include Pt, Pd, Rh, Au, Ru, Ir, or any combination thereof.

The support is substantially free of direct contact with the one or moremetal atoms. In some examples, the support includes SiO₂ and the metaloxide clusters include CeO_(x), CoO_(x), CuO_(x), FeO_(x), or anycombination thereof.

In a second general aspect, fabricating a nanocomposite catalystincludes forming nanoscale metal oxide clusters including a first metalon a support, and depositing one or more metal atoms including a secondmetal on the nanoscale metal oxide clusters.

Implementations of the second general aspect may include one or more ofthe following features.

The first metal and the second metal may be the same or different. Theone or more metal atoms may independently include one or more transitionmetal atoms, one or more precious metal atoms, or any combinationthereof. In some cases, the nanoscale metal oxide clusters includeCeO_(x), CoO_(x), FeO_(x) TiO_(x), CuO_(x), NiO_(x), MnO_(x), NbO_(x),VO_(x), ZrO_(x) (e.g., CeO₂, CO₃O₄, Fe₂O₃, TiO₂, CuO, NiO, MnO₂, Nb₂O₅,V₂O₅, ZrO₂), or any combination thereof. The support is typically freeor substantially free of direct contact with the second metal. In somecases, the support includes a refractory material having a surface areaof at least 50 m²/g or at least 100 m²/g.

In a third general aspect, catalyzing a reaction includes contacting thenanocomposite catalyst of the first general aspect with reactants,wherein the reaction comprises CO oxidation, water-gas-shift reaction,reforming of CO₂ and methanol, or oxidation of natural gas.

The details of one or more embodiments of the subject matter of thisdisclosure are set forth in the accompanying drawings and thedescription. Other features, aspects, and advantages of the subjectmatter will become apparent from the description, the drawings, and theclaims.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1C depict a single metal atom, two different single metalatoms, and a metal cluster, respectively, anchored onto the surface of areducible metal oxide nanoisland bound to the surface of a refractoryoxide support.

FIG. 2A depicts solution deposition of positively charged metalcomplexes onto the negatively charged surfaces of a refractory oxidesupport. FIG. 2B depicts formation of uniformly deposited metalcomplexes onto the surface of a refractory oxide support. FIG. 2Cdepicts formation of individually isolated reducible metal oxidenanoisland by a high temperature calcination process.

FIG. 3A depicts a process for solution deposition of negatively chargedmetal species onto the positively charged surfaces of the reduciblemetal oxide nanoisland, but not onto the negatively charged surfaces ofthe refractory oxide support. FIG. 3B depicts a single metal atom whichdeposits on the reducible metal oxide nanoisland which is supported onthe refractory oxide support after washing, drying, and calciningprocesses.

FIG. 4A shows a representative low-magnification high-angle annulardark-field-scanning transmission electron microscopy (HAADF-STEM) imageof CeO_(x) nanoislands uniformly distributed throughout ahigh-surface-area SiO₂ support. FIG. 4B shows a representativehigh-magnification HAADF-STEM image of CeO_(x) nanoislands.

FIG. 5 shows a histogram of the particle size distribution of theCeO_(x) nanoislands obtained by analyzing high resolution HAADF-STEMimages.

FIG. 6 shows powder X-ray diffraction (XRD) patterns obtained from 12 wt% CeO_(x) nanoislands supported on SiO₂ and 12 wt % CeO₂ nanoparticlessupported on SiO₂.

FIG. 7A shows Ce 3d X-ray photoelectron spectroscopy (XPS) spectrarecorded over SiO₂ supported CeO_(x) nanoislands. FIG. 7B shows Ce 3dXPS spectra recorded over SiO₂ supported CeO₂ nanoislands.

FIG. 8 shows Raman spectra obtained over SiO₂ supported CeO_(x) and CeO₂nanoislands.

FIG. 9A shows a representative low-magnification HAADF-STEM image ofCoO_(x) nanoislands uniformly distributed throughout thehigh-surface-area SiO₂ support. FIG. 9B shows a representativehigh-magnification HAADF-STEM image of CoO_(x) nanoislands.

FIG. 10A shows a representative low-magnification HAADF-STEM image ofCuO_(x) nanoislands uniformly distributed throughout thehigh-surface-area SiO₂ support. FIG. 10B shows a representativehigh-magnification HAADF-STEM image of CuO_(x) nanoislands.

FIG. 11A shows a representative low-magnification HAADF-STEM image ofFeO_(x) nanoislands uniformly distributed throughout thehigh-surface-area SiO₂ support. FIG. 11B shows a representativehigh-magnification HAADF-STEM image of FeO_(x) nanoislands.

FIG. 12A shows a representative low-magnification HAADF-STEM image of atypical Pt₁/CeO_(x)—SiO₂ single-atom catalyst revealing the absence ofPt nanoparticles and nanoclusters. FIG. 12B shows a representativehigh-magnification HAADF-STEM image of a typical Pt₁/CeO_(x)—SiO₂single-atom catalyst showing the absence of Pt nanoparticles andnanoclusters.

FIG. 13A shows a representative HAADF-STEM image of a typicalCeO_(x)—SiO₂ supported Pt cluster catalyst. FIG. 13B shows arepresentative HAADF-STEM image of a typical CeO_(x)—SiO₂ supported Ptnanoparticle catalyst.

FIG. 14A shows a representative low-magnification HAADF-STEM image of atypical Pd₁/CeO_(x)—SiO₂ single-atom catalyst showing the absence of Pdnanoparticles and nanoclusters. FIG. 14B shows a representativehigh-magnification HAADF-STEM image of a typical Pd₁/CeO_(x)—SiO₂single-atom catalyst showing the absence of Pd nanoparticles andnanoclusters.

FIG. 15 shows conversion rate of CO oxidation over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst and the CeO_(x)—SiO₂ supportcontrol.

FIG. 16 shows conversion rate of CO oxidation over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ before and after H₂ reduction treatment.

FIG. 17 shows conversion rate of CO oxidation over the 0.05 wt %Pd₁/CeO_(x)—SiO₂ before and after H₂ reduction treatment.

FIG. 18 shows conversion rate of CO oxidation over Au/CeO_(x)—SiO₂catalyst.

FIG. 19 shows a stability test of CO oxidation over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst.

FIG. 20 shows a stability test for CO oxidation over the 0.05 wt %Pd₁/CeO_(x)—SiO₂ single-atom catalyst.

FIG. 21 shows a stability test for CO oxidation over the Au/CeO_(x)—SiO₂catalyst.

FIG. 22 shows a conversion rate of water-gas-shift reaction (WGS) overthe 0.05 wt % Pt₁/CeO_(x)—SiO₂ single-atom catalyst.

FIG. 23 shows a stability test of WGS over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst.

FIG. 24 shows a conversion rate for methanol steam reforming over the0.2 wt % Pt/CeO_(x)—SiO₂ catalyst.

FIG. 25A depicts high-number density of Pt₁ atoms supported onCeO_(x)—SiO₂ nanoislands which are supported on high-surface-arearefractory material. FIG. 25B shows HAADF-STEM images of a typicalPt₁/CeO_(x)—SiO₂ single-atom catalyst showing spatial distribution ofthe CeO_(x) nanoislands. FIG. 25C shows the atomic structure of theCeO_(x) clusters. The Pt₁ atoms cannot be reliably identified due tolack of proper image contrast. FIG. 25D shows catalytic testing data forCO oxidation on various types of catalysts.

DETAILED DESCRIPTION

Extremely stable supported metal atom and cluster catalysts have beendeveloped by judicially integrating metal atoms (e.g., noble metalatoms), reducible metal oxides, and refractory high-surface-areasupports. Atomically dispersed metal atoms and clusters are stabilizedby use of nanoscale metal oxides (“nanoislands”) attached to refractorysupport materials. The reducible metal oxides serve as a binder toconfine the movement of supported metal atoms or clusters duringcatalytic reactions. The reducible nanoscale metal oxides not onlystabilize metal atoms and clusters during a catalytic reaction at hightemperatures but also provide desirable functions to enhance theactivity of a desired catalytic reaction. The nanoscale metal oxidestypically have a dimension (e.g., diameter or height) of 0.5 nm to 10nm. In some cases, the nanoscale metal oxides having a dimension of 0.5nm up to 3 nm are referred to as “nanoglues,” while the nanoscale metaloxides having a dimension of 3 nm to 10 nm are referred to as“nanoparticles” or “nanocrystals.”

The type of metal can be any transition metal (e.g., precious metal).Suitable metal oxides include CeO_(x) (e.g., CeO₂), CoO_(x) (e.g.,Co₃O₄), FeO_(x) (e.g., Fe₂O₃), TiO_(x) (e.g., TiO₂), CuO_(x) (e.g.,CuO), NiO_(x) (e.g., NiO), MnO_(x) (e.g., MnO₂), NbO_(x) (e.g., Nb₂O₅),ZrO_(x) (e.g., ZrO₂) combinations of these oxides, and other appropriatemeal oxides. A typical dimension for the nanoscale reducible metal oxide(e.g., diameter or height) is in a range of 0.5 nm to 10 nm. Suitablehigh-surface-area refractory support materials include SiO₂, Al₂O₃, MgO,ZrO₂, combinations of these oxides, and other appropriate supportmaterials (e.g., mullite, cordierites, or perovskites).

The utilization of such manufactured stable catalysts has been testedfor CO oxidation, water-gas-shift reaction, reforming of CO₂ andmethanol, oxidation of natural gas, and the like. The catalyst designand synthesis strategy described herein is schematically illustrated inFIGS. 1-3 .

FIG. 1A depicts a single metal atom 3 preferentially anchored onto thesurface of a reducible nanoscale metal oxide 2, which strongly bindsonto the surface of a high-surface-area refractory oxide support 1. Asused herein, “high surface area” generally refers to at least 50 m²/g orat least 100 m²/g. FIG. 1B depicts a single metal atom 3 and a differentsingle metal atom 4 which are preferentially anchored onto the surfaceof a reducible nanoscale metal oxide 2 that strongly binds onto thesurface of a refractory oxide support 1. The single metal atoms 3 and 4can be associated with each other or independently anchored onto thesurface of a reducible nanoscale metal oxide 2. FIG. 1C depicts acluster of metal atoms (e.g., 2 to 100 metal atoms) 5 which ispreferentially anchored onto the surface of a reducible nanoscale metaloxide 2 that strongly binds onto the surface of a high-surface-arearefractory oxide support 1. The cluster 5 can include a single metal, abimetallic alloy, or combinations of two or more metals. Suitablerefractory oxide supports include SiO₂, Al₂O₃, MgO, ZrO₂, otherappropriate materials, or any combination thereof (e.g., mullite andcordierite). Suitable nanoscale metal oxides include reducible oxidenanoclusters such as CeO_(x), CoO_(x), TiO_(x), FeO_(x), CuO_(x),MnO_(x), NbO_(x), ZrO_(x), other appropriate oxides, or any combinationthereof. The reducible oxide nanoclusters typically have a dimension(e.g., diameter or height) of 0.5 nm to 10 nm. In some cases,nanoclusters having a dimension of 0.5 nm up to 3 nm are referred to as“nanoglues,” while nanoclusters having a dimension of 3 nm to 10 nm arereferred to as “nanoparticles” or “nanocrystals.” Suitable metals forthe metal atoms include precious metals (e.g., Pt, Pd, Rh, Au, Ru, Ir,Ag, and the like), transition metals, or any appropriate metal or alloycluster with a total number of atoms in a range of 2 to 200 (e.g., 2 to100 or 3 to 10).

FIG. 2A depicts solution deposition of positively charged metalcomplexes 7 onto the surface of a high-surface-area refractory oxidesupport 1 that are negatively charged 6 due to the presence of surfacespecies in aqueous solution at the appropriate pH. FIG. 2B depictsformation of uniformly deposited metal complexes 8 onto the surfaces ofthe high-surface-area refractory support 1 via strong electrostaticadsorption processes. FIG. 2C depicts formation of individually isolatedreducible metal oxide nanoisland glue 2, by a high temperaturecalcination process, uniformly covering the surfaces of thehigh-surface-area refractory oxide support 1.

FIG. 3A depicts a process for preferential solution deposition of metalatoms 9 onto the surfaces of the reducible metal oxide nanoisland glue2, but not onto the surface of the high-surface-area refractory oxidesupport 1. By tuning the solution pH value, the refractory oxide support1 can be made negatively charged while the reducible metal oxidenanoisland glue 2 becomes positively charged. The negatively chargedmetal complex 9 then can typically only deposit onto the positivelycharged reducible metal oxide nanoisland glue 2 due to strongelectrostatic attraction but cannot typically deposit onto thenegatively charged refractory oxide support 1 due to strong repulsion.FIG. 3B depicts single metal atom 3 which deposits on the nanoscalemetal oxide 2 which are attached to the refractory oxide support 1 afterwashing, drying, and calcining processes.

The specific synthesis examples illustrated below follow the generalprinciples of the design strategy. Reducible metal oxide nanoislands areused as functional nanoglues. The reducible nanoscale metal oxides aresynthesized by a facile wet chemical synthesis route. Specifically,metal complexes are solution deposited onto the refractory supportsurfaces by a strong electrostatic adsorption method. High temperaturecalcination of the deposited species produces isolated individualnanoscale metal oxide islands strongly attached to the refractorysupport surfaces. The metal atoms and/or clusters are preferentiallydeposited onto the surfaces of the isolated individual nanoscale metaloxides but not onto the surfaces of the refractory support materials byfine tuning the solution pH so that the nanoscale metal oxide surfacesmaintain a surface charge that is opposite to that of the depositedmetal complexes, and the refractory support surfaces maintain a surfacecharge similar to that of the deposited metal complexes.

Atomically dispersed metal atoms and clusters are stabilized by use ofnanoscale metal oxides. The synthesis processes include dispersing metaloxide clusters (e.g., 1 nm to 2 nm CeO_(x) clusters) on a support (e.g.,SiO₂) and then depositing single metal atoms (e.g., Pt) onto the metaloxide clusters. Extremely stable supported metal atom and clustercatalysts can be prepared by judicially integrating metal atoms (e.g.,noble metal atoms), reducible metal oxide nanoglues, and refractoryhigh-surface-area supports. The use of reducible nanoscale metal oxidesstabilizes metal atoms and clusters during a catalytic reaction at hightemperatures and provides desirable functions (e.g., providing readilyavailable active surface and/or lattice oxygen species) to enhance theactivity of a desired catalytic reaction.

Specific examples of facile and scalable wet chemistry methods tomanufacture supported metal atom and cluster catalysts that areresistant to sintering, even at elevated temperatures and under variousgas environment, are described below. In some embodiments, reduciblemetal oxide nanoislands strongly glue the metal atoms/clusters to ahigh-surface-area refractory support which can resist sintering at hightemperatures. The zeta potential of different materials can be utilizedto preferentially deposit metal atoms or clusters only to the reduciblemetal oxide nanoislands. The synthesis process is low cost, scalable,and ready for large scale manufacturing.

CO oxidation is used as a probe reaction to evaluate the stability ofthe prepared supported metal atom and cluster catalysts. For aPt₁/CeO_(x)—SiO₂ single-atom catalyst (SAC) system, results demonstratethat the CeO_(x) clusters stabilize the Pt₁ single atoms during the COoxidation and also enhance the activity, presumably due to the redoxcapability of CeO_(x) clusters that facilitate CO oxidation.

EXAMPLES

Synthesis of CoO_(x)—SiO₂

180 mg of fumed SiO₂ powder (surface area of 278 m²/g) was dispersedinto 30 mL of water by sonication. 54 mg of hexamminecobalt(III)chloride was dissolved into 20 mL of ammonia solution (concentration ofNH₃·H₂O was 5 mol/L). Under rigorous stirring, the Co precursor wasquickly injected into the SiO₂ solution. The mixture was aged understirring for 1 h and then the precipitate was collected by vacuumfiltration. The resultant orange Co—SiO₂ precipitates were removed forair dry overnight at room temperature. The dried powder was ground witha pestle and annealed at 400° C. for 12 h in a muffle furnace to obtainthe dark-green CoO_(x)—SiO₂ powder.

Synthesis of CuO_(x)—SiO₂

180 mg of fumed SiO₂ powder was dispersed into 30 mL of water bysonication. 48 mg of copper(II) nitrate hydrate was dissolved into 20 mLof ammonia solution (the concentration of NH₃·H₂O was 5 mol/L). Underrigorous stirring, the Cu precursor solution was quickly injected intothe SiO₂ solution. The mixture was aged under stirring for 1 h and thenthe blue precipitate was collected by vacuum filtration. Then theresultant Cu—SiO₂ precipitates were removed for air dry overnight atroom temperature. The dried powders were ground with a pestle andannealed at 400° C. for 12 h in a muffle furnace to obtain the finaldark-green CuO_(x)—SiO₂ powder.

Synthesis of FeO_(x)—SiO₂

180 mg of fumed SiO₂ powder was dispersed into 50 mL of water bysonication. 40 mg of iron(III) nitrate was added into the SiO₂ solution.Under rigorous stirring, 0.2 mL of ammonia solution (the concentrationof NH₃—H₂O was 2 mol/L) was quickly injected to the mixture solution.The mixture solution was aged under stirring for 1 h and then the orangeprecipitate was collected by vacuum filtration. Then the resultantFe—SiO₂ precipitates were removed for air dry overnight at roomtemperature. The dried powder was ground with a pestle and annealed at400° C. for 1 h in a muffle furnace to produce the final orangeFeO_(x)—SiO₂ powder.

Synthesis of 0.05 wt % Pt₁/CeO_(x)—SiO₂ Single-Atom Catalyst

300 mg of CeO_(x)—SiO₂ powder was dispersed into 72 mL DI water undersonication for 20 min. Then the pH of the solution was adjusted to below4 by using HCl (0.1 mol/L). 530 L of platinum precursor solution (2.82mg/mL of Pt) was diluted into 50 mL DI water and the pH was adjusted tobelow 4. Under rigorous stirring, the Pt precursor solution was slowlypumped into the CeO_(x)—SiO₂ solution under stirring over 4 h. Afteraging under stirring for another 2 h, the precipitates were filteredusing vacuum filtration and washed with DI water 3 times to remove anynon-adsorbed ions and any other residue species. The resultantprecipitates were dried in air overnight and then were calcined in airat 600° C. for 12 h.

Synthesis of Reduced 0.05 wt % Pt₁/CeO_(x)—SiO₂ Single-Atom Catalyst

300 mg of CeO_(x)—SiO₂ powder was dispersed into 72 mL DI water undersonication for 20 min. Then the pH of the solution was adjusted to below4 by using HCl (0.1 mol/L). 530 μL of platinum precursor solution (2.82mg/mL of Pt) was diluted into 50 mL DI water and the pH value wasadjusted to below 4. Under rigorous stirring, the Pt precursor solutionwas slowly pumped into the CeO_(x)—SiO₂ solution under stirring over 4h. After aging under stirring for another 2 h, the precipitates werefiltered using vacuum filtration and washed with DI water 3 times toremove any non-adsorbed ions and any other residue species. Theresultant precipitates were dried in air overnight and then werecalcined in air at 600° C. for 12 h. Prior to catalytic CO oxidationreaction, the as-calcined catalyst was reduced in 10 sccm (standardcubic centimeter per minute) of 5% H₂/He at 300° C. for 1 h. Suchreduced Pt₁/CeO_(x)—SiO₂ SACs significantly improve CO oxidationactivity.

Table 1 shows specific reaction rates of Pt (mmol CO/(gPt*s)) atdifferent reaction temperatures.

TABLE 1 Specific reaction rates of Pt (mmol CO/(g_(Pt)*s)) at differentreaction temperatures Catalyst 150° C. 160° C. 170° C. E_(a) 0.05 wt %Pt₁/CeO_(x)—SiO₂ 9.2 14.9 23.6 −67.8 kJ/mol 0.05 wt % PtNPs/CeO_(x)—SiO₂ 0.46 1.1 1.8 −79.3 kJ/mol 0.05 wt % Pt/SiO₂ 0.045 0.0560.077  −140 kJ/mol CeO_(x)—SiO₂ (control) — — — — The specific rates ofPt₁ atoms of Pt nanoparticles were measured with feed gas of 1.0 vol. %CO, 4.0 vol. % O₂ and He balance, pressure was 0.1 MPa. The apparentactivation energy (E_(a)) of the 0.05 wt % Pt₁/CeO_(x)—SiO₂ single atomcatalyst is the lowest, indicating most active. The conversion of COover CeO_(x)—SiO₂ (control) was practically zero at the reactiontemperatures evaluated.Synthesis of 0.05 wt % Pd₁/CeO_(x)—SiO₂ Single-Atom Catalyst

300 mg of CeO_(x)—SiO₂ powder was dispersed into 72 mL DI water and thesolution was then sonicated for 20 min. Then the solution pH wasadjusted to below 4 using HCl (0.1 mol/L). 747 μL of palladium (II)chloride solution (2.0 mg/mL of Pd) was diluted into 50 mL DI waterwhile the solution pH was maintained below 4. Under rigorous stirring,the Pd precursor solution slowly pumped into the CeO_(x)—SiO₂ solutionunder stirring over 4 h. After aging under stirring for another 2 h, theresultant precipitates were filtered using vacuum filtration and washedwith DI water for 3 times to remove non-adsorbed ions or other residuespecies. The precipitates were then dried in air overnight and furthercalcined in air at 400° C. for 3 h.

Synthesis of 2 wt % Pt/CeO_(x)—SiO₂ Cluster Catalyst

300 mg of CeO_(x)—SiO₂ powders were immersed into 72 mL DI water undersonication for 20 min. The solution pH was maintained below 4 by usingHCl (0.1 mol/L). 2.12 mL of platinum precursor solution (contains 2.82mg/mL of Pt) was diluted into 50 mL DI water while maintaining the pHbelow 4. Under rigorous stirring, the Pt precursor solution was pumpedinto the CeO_(x)—SiO₂ solution over 4 h. After aging (under stirring)for another 2 h, the precipitates were filtered using vacuum filtrationand washed with DI water 3 times to remove non-adsorbed ions or otherresidue species. The precipitates were dried in air overnight at roomtemperature and were then calcined in air at 600° C. for 12 h. Finally,the 2 wt % Pt/CeO_(x)—SiO₂ powders were reduced in 5 vol % CO at 400° C.for 5 h to produce uniformly distributed Pt nanoclusters that areattached to the CeO_(x) nanoglues.

FIG. 4A shows a representative low-magnification high-angle annulardark-field-scanning transmission electron microscopy (HAADF-STEM) imageof CeO_(x) nanoislands 400 uniformly distributed throughout ahigh-surface-area SiO₂ support 402. FIG. 4B shows a representativehigh-magnification HAADF-STEM image of CeO_(x) nanoislands 400,revealing the shape, size and spatial distribution of the CeO_(x)nanoislands that are strongly attached to the SiO₂ surface 402.

FIG. 5 shows a histogram of the particle size distribution of theCeO_(x) nanoglues obtained by analyzing high resolution HAADF-STEMimages. The average size of the CeO_(x) nanoglues is 1.8 nm.

FIG. 6 shows powder X-ray diffraction (XRD) patterns obtained fromdefect-rich 12 wt % CeO_(x) nanoglues (<3 nm) supported on SiO₂ and fromstoichiometric 12 wt % CeO₂ nanoparticles (3-10 nm) supported on SiO₂.The sizes of the CeO_(x) nanoglues are too small to give observablepeaks in the XRD pattern while the larger CeO₂ nanocrystals show clearlyrecognizable peaks in the XRD pattern.

FIG. 7A shows Ce 3d X-ray photoelectron spectroscopy (XPS) spectrarecorded over SiO₂ supported CeO_(x) nanoglues. FIG. 7B show Ce 3d XPSspectra recorded over SiO₂ supported CeO₂ nanocrystals. The amount ofCe³⁺, reflecting the defect state, is more dominant in the CeO_(x)nanoglues (˜26%) than that in the CeO₂ nanoparticles (˜10%).

FIG. 8 shows Raman spectra obtained over SiO₂ supported CeO_(x)nanoglues (upper) and CeO₂ nanocrystals (lower). Compared to the peak at462 cm⁻¹ of CeO₂ nanoparticles (NPs), the corresponding peak of the 12wt % CeO_(x)—SiO₂ redshifts to 448 cm⁻¹ and becomes much broader,reflecting the increase in lattice constant due to their smaller sizesand the strong interaction between the CeO_(x) nanoglues and the SiO₂support.

FIG. 9A shows a representative low-magnification HAADF-STEM image ofCoO_(x) nanoislands 900 uniformly distributed throughout thehigh-surface-area SiO₂ support 902. FIG. 9B shows a representativehigh-magnification HAADF-STEM image of CoO_(x) nanoislands 900 showingthe shape, size, and spatial distribution of the CoO_(x) nanoislandsattached to the SiO₂ surfaces 902.

FIG. 10A shows a representative low-magnification HAADF-STEM image ofCuO_(x) nanoislands 1000 uniformly distributed throughout thehigh-surface-area SiO₂ support 1002. FIG. 10B shows a representativehigh-magnification HAADF-STEM image of CuO_(x) nanoislands 1000 showingthe shape, size, and spatial distribution of the CuO_(x) nanoislandsattached to the SiO₂ surfaces 1002.

FIG. 11A shows a representative low-magnification HAADF-STEM image ofFeO_(x) nanoislands 1100 uniformly distributed throughout thehigh-surface-area SiO₂ support 1102. FIG. 11B shows a representativehigh-magnification HAADF-STEM image of FeO_(x) nanoislands 1100 showingthe shape, size, and spatial distribution of the FeO_(x) nanoislandsattached to the SiO₂ surface 1102.

FIG. 12A shows a representative low-magnification HAADF-STEM image of atypical Pt₁/CeO_(x)—SiO₂ single-atom catalyst 1200 revealing the absenceof Pt nanoparticles and nanoclusters. FIG. 12B shows a representativehigh-magnification HAADF-STEM image of a typical Pt₁/CeO_(x)—SiO₂single-atom catalyst 1202 showing the absence of Pt nanoparticles andnanoclusters. Single Pt atoms cannot be reliably imaged because of thelack of image contrast since the atomic number differences between Ptand Ce is not large enough and that the thicknesses of the CeO_(x)nanoislands change rapidly.

FIG. 13A shows a representative HAADF-STEM image of a typicalCeO_(x)—SiO₂ supported Pt cluster catalyst 1300 revealing the presenceof many Pt nanoclusters 1302 with sizes ranging from ˜0.4 nm to 1.0 nm.FIG. 13B shows a representative HAADF-STEM image of a typicalCeO_(x)—SiO₂ supported Pt nanoparticle catalyst 1300 showing thepresence of many Pt nanoparticles 1304 with sizes ranging from ˜0.5 nmto 2.0 nm.

FIG. 14A shows a representative low-magnification HAADF-STEM image of atypical Pd₁/CeO_(x)—SiO₂ single-atom catalyst 1400 showing the absenceof Pd nanoparticles and nanoclusters. FIG. 14B shows a representativehigh-magnification HAADF-STEM image of a typical Pd₁/CeO_(x)—SiO₂single-atom catalyst 1400 showing the absence of Pd nanoparticles andnanoclusters. Single Pd atoms cannot be reliably imaged because of thelack of image contrast since the atomic number differences between Pdand Ce is small and that the thicknesses of the CeO_(x) nanoislandschange rapidly.

CO Oxidation Reaction

The CO oxidation reaction over the fabricated catalysts was conducted ina fixed-bed plug-flow reactor at atmospheric pressure. Typically, 30 mgof catalyst was used for each catalytic test. For CO oxidation, thereaction temperature was ramped up with a heating rate of 1° C./min. Thefeed gas, containing 1 vol % CO, 4 vol % O₂ balanced with He, passedthrough the catalytic bed at a flow rate of 10.0 mL/min (correspondingto weight hourly space velocity (WHSV) of 20,000 mL/g·h). Outlet gascomposition was measured by an online gas chromatograph (Agilent 7890A)equipped with a thermal conductivity detector (TCD).

FIG. 15 shows conversion rate of CO oxidation over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst and the CeO_(x)—SiO₂ supportcontrol. Reaction conditions: 1 vol % CO and 4 vol % O₂ balanced withHe, pressure=0.1 MPa, space velocity=20,000 mL/g·h. The high COoxidation activity originates from the Pt single atoms.

FIG. 16 shows conversion rate of CO oxidation over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ before and after H₂ reduction treatment. Reactionconditions: 1 vol % CO and 4 vol % O₂ balanced with He, pressure=0.1MPa, SV=20,000 mL/g·h. The H₂ reduction process removes some oxygenligands which are weakly bound to the Pt atoms. The reduction of theoxidation state of the Pt atoms promoted their CO oxidation activity.The modification of the CeO_(x) by the H₂ reduction treatment may affectthe observed CO oxidation activity as well.

FIG. 17 shows conversion rate of CO oxidation over the 0.05 wt %Pd₁/CeO_(x)—SiO₂ before and after H₂ reduction treatment. Reactionconditions: 1 vol % CO and 4 vol % O₂ balanced in He, pressure=0.1 MPa,SV=20,000 mL/g·h. The calcined 0.05 wt % Pd₁/CeO_(x)—SiO₂ single-atomcatalyst is intrinsically active for CO oxidation. The H₂ reductionprocess did not appreciably change the CO oxidation activity.

FIG. 18 shows conversion rate of CO oxidation over Au/CeO_(x)—SiO₂catalyst. Reaction conditions: 1 vol % CO and 4 vol % O₂ balanced withHe, pressure=0.1 MPa, SV=20,000 mL/g·h.

FIG. 19 shows a stability test of CO oxidation over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst. The CO oxidation reaction wasconducted at 140° C. The 0.05 wt % Pt₁/CeO_(x)—SiO₂ single-atom catalystdid not show any trend of deactivation during the long-term test.Reaction conditions: 1 vol % CO and 4 vol % O₂ balanced with He,pressure=0.1 MPa, SV=20,000 mL/g·h.

FIG. 20 shows a stability test for CO oxidation over the 0.05 wt %Pd₁/CeO_(x)—SiO₂ single-atom catalyst. The CO oxidation reaction wasconducted at 120° C. Reaction conditions: 1 vol % CO and 4 vol % O₂balanced with He, pressure=0.1 MPa, SV=20,000 mL/g·h.

FIG. 21 shows a stability test for CO oxidation over the Au/CeO_(x)—SiO₂catalyst. The CO oxidation reaction was conducted at 80° C. Reactionconditions: 1 vol % CO and 4 vol % O₂ balanced with He, pressure=0.1MPa, SV=20,000 mL/g·h.

Water-Gas-Shift Reaction (WGS)

The WGS reaction over the fabricated catalysts was conducted in afixed-bed plug-flow reactor at atmospheric pressure. Typically, 30 mg ofcatalyst was used for each test. Catalyst powders were pretreated in 10sccm (standard cubic centimeter per minute) of 5% H₂/He at 300° C. for 1h. The reaction temperature was ramped up with a heating rate of 2°C./min. The feed gas, containing 1 vol % CO balanced with He, passedthrough a water reservoir which was heated to 33° C. The gas mixturewent through the catalytic bed at a flow rate of 10.0 mL/min(corresponding to a weight hourly space velocity (WHSV) of 20,000mL/g·h). Outlet gas composition was measured by an online gaschromatograph (Agilent 7890A) equipped with a thermal conductivitydetector (TCD).

FIG. 22 shows conversion rate of WGS reaction over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst. Reaction conditions: 1 vol % COand 5 vol % H₂O balanced with He, pressure=0.1 MPa, spacevelocity=20,000 mL/g·h.

FIG. 23 shows a stability test of WGS over the 0.05 wt %Pt₁/CeO_(x)—SiO₂ single-atom catalyst. The WGS reaction was conducted at300° C. Reaction conditions: 1 vol % CO and 5 vol % H₂O balanced withHe, pressure=0.1 MPa, space velocity=20,000 mL/g·h.

Methane Combustion Reaction

The methane combustion reaction over the fabricated catalysts wasconducted in a fixed-bed plug-flow reactor at atmospheric pressure.Typically, 30 mg of catalyst was used for each test. Before reactiontest, the catalyst was pretreated with 10 sccm (standard cubiccentimeter per minute) of 5% H₂/He at 300° C. for 1 h. For methanecombustion, the reaction temperature was ramped up with a heating rateof 1° C./min. The feed gas, containing 1 vol % CH₄, 4 vol % O₂ balancedwith He, passed through the catalytic bed at a flow rate of 10.0 mL/min(corresponding to a weight hourly space velocity of 20,000 mL/g·h).Outlet gas composition was measured by an online gas chromatograph(Agilent 7890A) equipped with a thermal conductivity detector (TCD).

Methanol Reforming Reaction

The methanol reforming reaction over the synthesized catalysts wasconducted in a fixed-bed flow reactor at atmospheric pressure.Typically, 30 mg of catalyst was used for each test. Before reaction,the catalyst was pretreated with 10 sccm (standard cubic centimeter perminute) of 5% H₂/He at 300° C. for 1 h. For methanol reforming reaction,the reaction temperature was ramped up with a heating rate of 1° C./min.The feed gas, containing 10 vol % CH₃OH, 7 vol % H₂O balanced with He,passed through the catalytic bed at a flow rate of 10.0 mL/min(corresponding to weight hourly space velocity of 20,000 mL/g·h). Outletgas composition was measured by an online gas chromatograph (Agilent7890A) equipped with a thermal conductivity detector (TCD).

FIG. 24 shows a conversion rate for methanol steam reforming over the0.2 wt % Pt/CeO_(x)—SiO₂ catalyst. Reaction conditions: 1 vol % methanoland 1 vol % H₂O balanced with He, pressure=0.1 MPa, spacevelocity=20,000 mL/g·h.

Synthesis of CeO_(x)—SiO₂

CeO_(x) clusters were used as nanoglues to anchor Pt single atoms ontohigh surface area, inexpensive, and abundant SiO₂ supports, as depictedin FIG. 25A. The highly reducible CeO_(x) clusters act as an oxygengateway to store/release oxygen during selected catalytic reactions. COoxidation was used as a probe reaction to test the catalytic performanceof Pt₁/CeO_(x) clusters dispersed onto SiO₂ nanoparticles.

180 mg of fumed SiO₂ powder (surface area of 278 m²/g) was mixed with 50mL of water and then sonicated to obtain a uniform suspension. Underrigorous stirring, 86 mg of Ce(NO₃)₃.6H₂O was added into the SiO₂solution. Subsequently, 0.4 mL of NH₃—H₂O (concentration 2 mol/L) wasquickly injected into the mixed solution. After stirring for 3 min, themixture was collected by vacuum filtration. The resultant light-brownCe—SiO₂ precipitate was dried in air overnight at room temperature. Thedried powder was ground with a pestle and then annealed at 600° C. for12 h in a muffle furnace to obtain the light-yellow colored CeO—SiO₂powders. The loading of the CeO_(x) was 12 wt % by inductively coupledplasma mass spectrometry (ICP-MS) measurement. Through the sameprocedure, the 6 wt % CeO—SiO₂ was synthesized by using 43 mg ofCe(NO₃)₃.6H₂O and 0.2 mL of ammonia (2 mol/L). This synthesis processwas successfully scaled up to 10 times, in which 1800 mg of SiO₂, 500 mLof H₂O, 860 mg of Ce(NO₃)₃.6H₂O and 4 mL of NH₃—H₂O were used,respectively.

A strong electrostatic adsorption method was used to disperse Pt saltprecursors onto the surfaces of the as-prepared CeO_(x)—SiO₂nanocomposite powders. The Pt/CeO—SiO₂ precipitates were then filtered,washed and dried at 60° C. for 5 h. The Pt₁/CeO_(x)—SiO₂ powders, with anominal loading of 0.05 wt. % of Pt, were calcined and/or reduced toform the final Pt₁ SACs.

FIG. 25A depicts high-number density CeO_(x)—SiO₂ supported Pt₁ catalyst2500, with platinum atoms 2502 on CeO_(x) cluster 2504 and SiO₂ support2506. FIG. 25B shows HAADF-STEM images of a typical Pt₁/CeO_(x)—SiO₂single-atom catalyst showing spatial distribution of the CeO_(x)nanoislands 2508. FIG. 25C shows the atomic structure of the CeO_(x)clusters 2510. The Pt₁ atoms cannot be reliably identified due to lackof proper image contrast. FIG. 25D shows catalytic testing data for COoxidation. The conversion rates for the Pt₁/SiO₂ and the CeO_(x)—SiO₂controls were displayed as well. Reaction conditions: 1 vol % CO and 4vol % O₂ balanced with He, pressure=0.1 MPa, SV=20,000 mL/g·h.

FIG. 25B shows representative HAADF-STEM images of the as-synthesizedPt₁/CeO_(x)—SiO₂ SAC. The bright patches (indicated by the yellowarrows) represent uniformly dispersed CeO_(x) clusters with an averagesize of 2 nm and Pt nanoclusters are not observable. DRIFTS and EXAFSresults show that the Pt species exists as isolated single Pt atoms. TheCe 3d XPS spectrum of the Pt₁/CeO_(x)—SiO₂ SAC reveal enrichment ofoxygen vacancies in the CeO_(x) nanoclusters since the Ce³⁺/(Ce⁴⁺+Ce³⁺)ratio is 26% (FIG. 7A) vs 10% for CeO₂ nanocrystals (FIG. 7B). COoxidation over the as-synthesized Pt₁/CeO_(x)—SiO₂ SAC demonstrates thatthe CeO_(x) anchored Pt₁ atoms are not only highly active but extremelystable during the three reaction cycles (FIG. 25D). The CO-DRIFTSresults of the used Pt₁/CeO_(x)—SiO₂ SAC demonstrate that the localizedPt₁ atoms did not sinter and maintained their catalytic property duringthe CO oxidation reaction. The crystalline nature and the dispersion ofthe CeO_(x) clusters are clearly shown in FIG. 25C. Some small CeO_(x)clusters are almost atomically dispersed and do not possess acrystalline phase. Although it is difficult to distinguish single Ptatoms from those of the highly dispersed Ce atoms/clusters we have notdetected, in the as-prepared and used SACs, any Pt particles/clusterswhich are distinguishable from the CeO_(x) clusters. FIG. 25D shows thecatalytic testing data clearly demonstrating the high activity andstability of the Pt₁/CeO_(x)—SiO₂ SAC for the CO oxidation reaction.

Although this disclosure contains many specific embodiment details,these should not be construed as limitations on the scope of the subjectmatter or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particular embodiments.Certain features that are described in this disclosure in the context ofseparate embodiments can also be implemented, in combination, in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments, separately, or in any suitable sub-combination. Moreover,although previously described features may be described as acting incertain combinations and even initially claimed as such, one or morefeatures from a claimed combination can, in some cases, be excised fromthe combination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Particular embodiments of the subject matter have been described. Otherembodiments, alterations, and permutations of the described embodimentsare within the scope of the following claims as will be apparent tothose skilled in the art. While operations are depicted in the drawingsor claims in a particular order, this should not be understood asrequiring that such operations be performed in the particular ordershown or in sequential order, or that all illustrated operations beperformed (some operations may be considered optional), to achievedesirable results.

Accordingly, the previously described example embodiments do not defineor constrain this disclosure. Other changes, substitutions, andalterations are also possible without departing from the spirit andscope of this disclosure.

What is claimed is:
 1. A nanocomposite catalyst comprising: a support; amultiplicity of nanoscale metal oxide clusters coupled to the support;and 1-100 metal atoms coupled to each of the nanoscale metal oxideclusters, wherein the support is negatively charged, the nanoscale metaloxide clusters are positively charged, the 1-100 metal atoms arenegatively charged, and the support is free of direct contact with the1-100 metal atoms.
 2. The catalyst of claim 1, wherein the supportcomprises a refractory material having a surface area of at least 50m²/g or at least 100 m²/g.
 3. The catalyst of claim 2, wherein thesupport comprises silica, alumina, magnesia, zirconia, cordierite,mullite, perovskite or any combination thereof.
 4. The catalyst of claim2, wherein the support is powdered.
 5. The catalyst of claim 1, whereinthe nanoscale metal oxide clusters comprise CeO₂, Co₃O₄, Fe₂O₃, TiO₂,CuO, NiO, MnO₂, Nb₂O₅, ZrO₂ or any combination thereof.
 6. The catalystof claim 1, wherein the 1-100 metal atoms independently comprise one ormore transition metal atoms.
 7. The catalyst of claim 6, wherein the1-100 metal atoms independently comprise one or more precious metalatoms.
 8. The catalyst of claim 7, wherein the 1-100 metal atomscomprise Pt, Pd, Rh, Au, Ru, Ir, or any combination thereof.
 9. Thecatalyst of claim 1, wherein the support comprises SiO₂ and the metaloxide clusters comprise, CeO₂, Co₃O₄, CuO, Fe₂O₃, or any combinationthereof.
 10. The catalyst of claim 1, wherein the nanoscale metal oxideclusters have a dimension in a range of 0.5 nm to 10 nm.
 11. A method ofcatalyzing a reaction comprising contacting the nanocomposite catalystof claim 1 with reactants, wherein the reaction comprises CO oxidation,water-gas-shift reaction, reforming of CO₂ and methanol, or oxidation ofnatural gas.
 12. A method of fabricating a nanocomposite catalyst, themethod comprising: forming nanoscale metal oxide clusters comprising afirst metal on a support; and depositing 1-100 metal atoms comprising asecond metal on the nanoscale metal oxide clusters, wherein the supportis free of direct contact with the second metal.
 13. The method of claim12, wherein the nanoscale metal oxide clusters comprise CeO₂, Co₃O₄,Fe₂O₃, TiO₂, CuO, NiO, MnO₂, Nb₂O₅, ZrO₂, or any combination thereof.14. The method of claim 12, wherein the 1-100 metal atoms independentlycomprise one or more transition metal atoms.
 15. The method of claim 12,wherein the 1-100 metal atoms independently comprise one or moreprecious metal atoms.
 16. The method of claim 12, wherein the supportcomprises a refractory material having a surface area of at least 50m²/g or at least 100 m²/g.